1. Field of the Invention
The present invention relates to metal alloys, and particularly to a corrosion-resistant nickel-base alloy that has an outstanding combination of corrosion resistance, mechanical strength, and thermal stability.
2. Description of the Related Art
In the middle to late 1920's, it was observed that the corrosion resistance of nickel in reducing environments, particularly in hydrochloric acid, is greatly improved by additions of more than 15-weight percent molybdenum. Molybdenum also improves the corrosion resistance of nickel-base alloys in other nonoxidizing solutions, such as acetic and phosphoric acids. Since then, efforts during the last nine decades to materialize this observation into useful engineering products has led to the development of three major commercial alloys, which are listed in Table 1. Such alloys can find many important applications in the chemical process and petrochemical industries. The first alloy of commercial grade was introduced around 1929 as alloy B (U.S. Pat. No. 1,710,445). As described below, the composition of the alloy was adjusted at various stages of its history to improve fabricability and service performance until it became known as Hastelloy alloy B (Table I).
TABLE INominal Chemical Composition (weight %)HastelloyHastelloyHastelloyElementAlloy BAlloy B-2Alloy B-3NiBalanceBalanceBalanceMo28   28   28.5Cr1.0*1.0*1.5Fe5*  2*  1.5W——3.0*Co2.5*1.0*3.0*Si1.0*0.1*0.1*Mn1.0*1.0*3.0*Al——0.5*Ti——0.2*C 0.05* 0.01*0.01**Maximum
To facilitate manufacturing and reduce costs, the initial version of alloy B contained relatively high concentrations of iron and carbon, and other impurities, particularly silicon and manganese. At that time, although the alloy could be fabricated by casting, the production of wrought products was impaired by poor hot workability due to the presence of copper. This problem was solved in the 1940's by reducing the copper content while maintaining the same level of corrosion resistance.
Another problem was encountered in welded components due to the formation of secondary phases at grain boundaries of the weld heat-affected zone, which degraded the corrosion resistance. However, the nature of these precipitates could not be identified until the late fifties and early sixties when it was determined that the intergranular precipitates degrading the corrosion resistance were Mo-rich carbides. At that time, it was not possible to reduce the carbon content to a level preventing the formation of carbides. Therefore, in order to eliminate the need for a post-welding heat treatment and to maintain corrosion resistance in the as-welded condition, more emphasis was placed upon reducing the contents of other elements, particularly iron and silicon, which increase the solubility of carbon.
In the meantime, it was thought that any excess carbon could be stabilized by refractory elements, particularly niobium and vanadium. In the mid-sixties, the Hastelloy alloy B-282 was introduced; however, its properties proved to be inferior to those of the Hastelloy alloy B. In the early seventies, it became possible to produce alloys with very low carbon content, which resulted in the development of what is now known as Hastelloy alloy B-2 (Table 1), which is free of vanadium and has low concentrations of silicon and manganese. However, alloy B-2 was then found to be prone to long-range ordering reactions during short-term exposure at temperatures in the range of 600-800° C. In addition to degrading mechanical strength, the phase(s) resulting from these reactions were also found to have adverse effects on corrosion resistance.
Attempts to improve the thermal stability of alloy B-2 led to the development of Hastelloy alloy B-3 (see Table I). Although alloy B-3 has a somewhat higher thermal stability than B-2, it relies only upon slightly decelerating the kinetics of detrimental long-range ordering reactions at temperatures in the range of 600-800° C.
To guard against catastrophic failure resulting from extended exposure at the critical temperatures, there is a market need for another alloy with high molybdenum content to fulfill the requirements of relevant industries, and which is characterized by: i) an extended incubation period for ordering, and ii) an acceptable combination of properties, even in the ordered state.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a corrosion-resistant nickel-base alloy solving the aforementioned problems is desired.